Sewage plants produce large amounts of sewage sludge. To reduce the volume of sludge to be disposed of it is often collected and subjected to anaerobic digestion in tanks with floating horizontal covers which are vertically movable with change in volume of liquid, gas, or both, beneath the cover.
In one type of floating cover tank used to collect and treat sludge, the cover floats directly on the liquid sludge and gas produced by the digesting sludge is removed through a gas take-off housing and vented to the atmosphere. Another type of tank in which liquid sludge is anaerobically digested utilizes what is often referred to as a gasholder cover. The gasholder cover traps gas, produced in the sludge digestion process, beneath the cover thereby enabling it to float above the liquid sludge on the variable volume gas bubble.
In each of the described types of tank, the horizontally positioned floating cover is vertically movable with change in volume of liquid, gas, or both, beneath the cover. To aid in vertical movement of the cover it has been conventional practice to utilize metal rollers. In some tanks, the rollers have been mounted on the cover edge so as to contact the tank wall and/or one or more guide tracks on the tank. In other tanks, particularly those having gasholder covers, rollers have been mounted on the tank wall so that they engage vertical guide tracks on the cover periphery. The environmental conditions where the rollers are located lead to corrosion of the bearings, whether they be ball, roller or sleeve bearings, so that the rollers tend to seize to the supporting shaft. The rollers, being fixed and unable to roll, slide along the guide track or other surface on which they were intended to roll. Because of the small wear area of the rollers and high loads applied per unit area, the rollers wear quickly and vertical movement of the cover becomes restricted. A need accordingly exists for an improved floating cover tank with cover guides which avoids or minimizes the problems inherently present in the systems now in use.